Fluid flow sensor and fluid flow measurement device

ABSTRACT

A Thermal type fluid flow sensor comprises a heating resistor formed on a thin film of a substrate, and plural thermal sensitive resistors configuring a bridge circuit. The thermal sensitive resistors are disposed on the thin film of the substrate so as to be located on an adjacent upstream side and an adjacent downstream side of the heating resistor in a stream direction of fluid to be measured. Resistor traces for the thermal sensitive resistors are formed so that the respective thermal sensitive resistors exhibit substantially equal changes in resistance with each other to distortion caused in the thin film.

CLAIM OF PRIORITY

The present application claims priority from Japanese application serialno.2004-280804, filed on Sep. 28, 2004, the contents of which are herebyincorporated by references into this application.

BACKGROUND OF THE INVENTION

The present invention relates to a fluid flow sensor for measuring afluid flow rate by using a heating resistor and, for example, it relatesto a fluid flow measurement device suitable to a measurement for anintake airflow rate of an internal combustion engine.

Of various types of the fluid flow sensors that are, for example,disposed in various intake air passages of internal combustion enginesin automobile cars etc., a thermal type fluid flow sensor has come intowidespread use because of their capability to directly sense mass airflow rate.

In such thermal type fluid flow sensors, especially, those by using asensor elements manufactured by semiconductor micromachining technology,which are provided on the semiconductors substrate such as silicon (Si),are advantageous in terms of economical mass-producibility, andlow-power driving with size reduction. Therefore, thermal type fluidsensor using a sensor element based on this semiconductor technology hasgained the spotlight in recent years. Such fluid flow sensors aredescribed in Japanese Patent Laid-Open No. 2002-48616 (Patent Document1).

In the fluid flow sensor described in the Patent Document 1, sensingresistors as sensing elements are formed on a silicon substrate byinterposing an insulating layer between the heating resistor and thethin film. In such a manufacturing process, a portion of the siliconsubstrate is removed to form a thin film (diaphragm portion) forthermally insulating the resistance. A heating resistor is formed on thethin film to be driven as a heater. Plural thermal sensitive resistorsfor measuring fluid flow is disposed on the thin film of the substrateso as to be located on an adjacent upstream side and an adjacentdownstream side of the heating resistor in a stream direction of fluidto be measured. Fluid flow rate is sensed by a measuring difference oftemperature between the upstream side thermal sensitive resistor and thedownstream side thermal sensitive resistor. In the measuring methodbased on difference of temperature, the heating resistor is heated at aconstant temperature and heating the thermal sensitive resistors at theupstream and at the downstream by heat conduction and heat transfer. Ina case where the air flow rate is not present, the upstream side thermalsensitive resistor and the downstream side thermal sensitive resistorare theoretically heated identically, and the temperature differencetherebetween is substantially zero. When the air flow rate is present,since the upstream side thermal sensitive resistance is cooled and thetemperature is lowered, but the downstream side thermal sensitiveresistance is little cooled because heated air flows through the downstream side resistance. Accordingly a temperature difference occursbetween them. Since the temperature difference corresponds the air flowrate, the air flow rate can be sensed based on the amount of thetemperature difference. Since the upstream side and downstream sidethermal sensitive resistors change their resistance values in accordancewith the respective temperatures, voltage signals in accordance with theflow rate can be obtained by utilizing the change of the resistancevalues.

As shown in FIG. 1 and FIG. 6 of the Patent Document 1, by formingthermal sensitive resistors at the upstream and the downstream each bytwo pairs to form a bridge circuit, the sensor sensitivity can bedoubled. Further, the measuring method based on the difference oftemperature can detect the flowing direction of air. In a case where theair flow rate occurs in the direction opposite to that described above,the thermal sensitive resistor at the downstream is cooled. Accordingly,the circuit constitution shown in FIG. 6 of Patent Document 1 generatesan output in the direction opposite to the point of zero flow rate. Bythe provision of the direction detecting means, the air flow rate can bemeasured more accurately than the fluid flow sensor not having thedirection detecting means in a running state of causing an air flow inthe directing from an engine to an air cleaner (reverse flow). In theprior art, pulsation of intake air increases at a low speed of4-cylinder engines to often cause reverse flow near the full open stateof a throttle. However, pulsation and reverse flow tend to occur at highspeed to increase amount of the reverse flow in engine adopted forcomplicate control such as change of vale on-off time for coping withexhaust gas regulation and requirement for reducing fuel cost in recentyears. Further, pulsative flow including reverse flow occurs also in afour or more multi-cylinder engine. Accordingly, the direction detectingfunction is an extremely effective means.

Further, thermal response at high speed to flow rate change can beobtained by forming such flow-rate measurement resisters on the thinfilm as diaphragm. In a case where a high-response fluid flow sensor isapplied to the control of an automobile, it can response to the abruptchange of flow rate, or to the occurrence of pulsation in an air intakepipe. Accordingly, it is possible to measure the air flow rate moreaccurately than the fluid flow sensor of large heat capacity and at slowresponse speed.

The fluid flow sensor described above comprises mainly a flow sensingelement, a basis for mounting the flow sensing element, a circuit fordriving the flow sensing element and a case for mounting componentsdescribed above and attached to an intake pipe for flowing intake air,and the flow sensing element is disposed in a bypass passage as asecondary passage.

SUMMARY OF THE INVENTION

In Patent Document 1, polycrystalline silicon is used as resistors forthe flow sensing elements. A semiconductor material such aspolycrystalline silicon has a piezoresistance effect that the resistancevalue changes due to distortion occurred by the deformation of theshape. The amount of the piezoresistance effect is determined by a gaugefactor inherent to the material and this is found also in a metalmaterial such as platinum.

The prior art involves a problem that the output abnormality due to thepiezoresistance effect tends to occur easily. Since the thin filmdescribed above of the flow sensing element has only about 1 to 2 μmthickness, it undergoes various deformations due to:

-   (1) stress generated upon bonding the flow sensing element to a    basis,-   (2) stress generated upon bonding and mounting the basis to the case    or the like,-   (3) stress generated depending on the difference of linear expansion    coefficient of mounted materials due to the change of the    environmental temperature,-   (4) thermal deformations due to heating resistor.

Particularly, the thermal sensitive resistors formed on adjacentupstream and downstream sides of the heating resistor have to be formedwith narrow width and large length in view of the shape. Because it isdesirable that the resistance value thereof is high in view ofperformance. Accordingly, they tend to suffer from the piezoresistanceeffect due to the stresses described above. For example, in FIG. 6 ofPatent Document 1, while thermal sensitive resistors are formed each bytwo pairs on the upstream and the downstream, and bridge is formed byfour temperature measurement resistances. However, since fourresistances show different change of resistance values respectively dueto the deformation of the thin film, output abnormality tends to occur.

Particularly, in recent years, measurement is necessary as far as anextremely low flow rate for lowering idling with an aim of decreasingthe fuel cost and the output abnormality is particularly remarkable inthe low flow rate region.

An object of the present invention is to decrease the fluctuation ofsignals from a bridge circuit in which thermal sensitive resistors areconnected, even when the distortion occurs in the thin film of asubstrate on which the heating resistor and the thermal sensitiveresistor are disposed.

The foregoing object is attained in accordance with the inventionsdescribed in the claims.

For example, the foregoing object can be attained by a fluid flow sensorcomprising: a heating resistor formed on a thin film of a substrate;plural thermal sensitive resistors configuring a bridge circuit andbeing disposed on the thin film of the substrate so as to be located onan adjacent upstream side and an adjacent downstream side of the heatingresistor in a stream direction of fluid to be measured; wherein resistortraces as pattern elements for the thermal sensitive resistors areformed so that the respective thermal sensitive resistors exhibitsubstantially equal changes in resistance with each other to distortioncaused in the thin film.

According to the invention, in a case where distortion occurs in thethin film of the substrate for the heating resistor or thermal sensitiveresistor, fluctuations of the signals from the bridge circuit, in whichthe thermal sensitive resistors are connected, caused by the distortioncan be decreased.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a pattern diagram of a flow sensing element showing a firstexample of the invention.

FIG. 2 is a cross sectional view of a flow sensing element showing afirst example of the invention.

FIG. 3 is a fragmentary cross sectional view of a fluid flow sensormodule mounting the flow sensing element of the invention.

FIG. 4 is a fragmentary cross sectional view of a fluid flow sensormodule mounting the flow sensing element of the invention.

FIG. 5 is a driving circuit diagram in the invention.

FIG. 6 is a schematic view showing the temperature distribution, on athin film, of a heating resistor of the invention.

FIG. 7 is a pattern diagram of an existent flow sensing element.

FIG. 8 is an existent driving circuit diagram.

FIG. 9 is a pattern of thermal sensitive resistors of the invention.

FIG. 10 is a pattern diagram of a flow sensing element showing a secondexample of the invention.

FIG. 11 is a pattern diagram of a flow sensing element showing a thirdexample of the invention.

FIG. 12 is a system chart for an internal combustion engine using afluid flow sensor of the invention.

FIG. 13 is a pattern chart of a flow sensing element showing a fourthexample of the invention.

FIG. 14 is a cross sectional view of a flow sensing element showing afourth example of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

At first, a concept of an embodiment of the present invention is to bedescribed briefly. Resistor traces as pattern elements for the thermalsensitive resistor formed to the thin film has a configuration capableof reducing the change (variation) of output due to the piezoresistanceeffect. In a case where the resistor trace is configured by connectingstraight line potions in two or more directions each of an identicalwidth, it is configured so that the sum for lengths of trace potions issubstantially equal between each of the directions. Further, in a casewhere the pattern is configured by connecting straight line potions intwo or more directions of different pattern width, it is configured sothat L/W in each of the directions is substantially identical, assumingthe pattern width in each of the directions as W and the sum for thetrace length in each of the directions as L. With such a configuration,since the amount of change of resistance value is equal between each ofthe thermal sensitive resistors in a case where distortion occurs due tothe stress in the flow sensing element, it can provide a configurationof less changing the output of the fluid flow sensor.

Then, embodiments of the present invention will be describedspecifically with reference to the drawings.

A flow sensing element 1 used in the present invention is to bedescribed. FIG. 1 is a pattern diagram of a flow sensing element 1showing a first embodiment of the present invention, FIG. 2 shows across sectional view of the flow sensing element 1 showing the firstembodiment. The flow sensing element 1 is manufactured by asemiconductor production technique. This is to be described below. Asilicon dioxide layer is formed as an insulating layer 3 on a singlecrystal silicon substrate 2 by way of a method such as thermal oxidationor CVD (Chemical Vapor Deposition), and a silicon nitride layer isformed by a CVD or like other method. Then, a polycrystalline siliconlayer is formed by a method of CVD or the like, and phosphorus (P) asimpurities is doped by thermal diffusion or ion implantation in order toobtain a desired resistance value. Subsequently, the polycrystallinesilicon layer is patterned to form a heating resistor 4, an intakeair-temperature compensating resistor 5, upstream side thermal sensitiveresistors 6, 7, downstream side thermal sensitive resistors 8, 9, andlead resistance 12. Then, a silicon nitride layer and a silicon dioxidelayer are formed as a protective layer 13 by a method such as CVD.Subsequently, the protective layer 13 is patterned to remove theprotective layer 13 at a portion for forming electrodes 14. Then, afterforming an aluminum layer as an electrode material, patterning isapplied by etching to form electrodes 14. Finally, for forming a cavity15, patterning is applied to the surface of the single crystal siliconsubstrate 2 on the surface not formed with the heating resistor 4. Then,the cavity 15 is formed by anisotropic etching. By forming the cavity, aregion in which the heating resistor 4, the upstream side thermalsensitive resistors 6, 7 and the downstream side thermal sensitiveresistors 8, 9 are disposed becomes a thermally insulated thin film 16like a diaphragm. Since the silicon dioxide and polycrystalline siliconcontain compressive stress and silicon nitride contains tensile stress,a thin film 16 with no distortion can be formed by stacking thematerials each of an appropriate film thickness. Finally, it is dividedinto chips by dicing. The divided flow sensing element 1 has, forexample, of about 5 mm longer side and about 2.5 mm of shorter side in arectangle. Further, by the use of a substrate formed by cementing twosheets of single crystal silicon substrates oxidized at the surface,those resistors can also be formed by single crystal silicon.

Then, a configuration of a fluid flow sensor 100 is to be described withreference to FIG. 3 and FIG. 4. FIG. 3 and FIG. 4 are partial crosssectional views of a fluid flow sensor module in which the flow sensingelement 1 according to the present invention is mounted. A basis 20 formounting the flow sensing element 1 is formed by a laminated substratemade of glass ceramics. As the basis 20, high temperature calcinationceramics, metal plates, etc. may also be used. Especially, since it isdesirable that the flow sensing element 1 is thermally insulated fromperipheral members, use of a laminated substrate made of glass ceramicsof low heat conductivity is effective. Further, by the use of thelaminated substrate, a circuit 21 for feeding power to the flow sensingelement 1, and processing signals from the flow sensing element 1 can beformed integrally in the basis 20. Since the number of parts can bedecreased and the portion for bonding can be decreased by integratingthe basis 20 and the circuit 21, it is advantageous in view of the costreduction and the reliability. Further, since the size of the circuit 21can be reduced by constituting the circuit 21 for controlling the flowsensing element 1 by using an inner layer conductor of the laminatedsubstrate, the size of the fluid flow sensor 100 can be decreased.

The flow sensing element 1 is bonded on the basis 20 by way of an epoxyor silicone adhesive 22, and the electrode 14 of the flow sensingelement 1 and the electrode of the basis 20 are electrically connected,for example, by connection wires such as gold wire 23, etc. Theconnection portion is covered with an epoxy or silicone resin so as toprevent electrolytic corrosion due to contaminants or moisture containedin intake air. The basis 20 on which the flow sensing element 1 ismounted is mounted by means of silicone adhesive 23 to a housing case24. Further, the housing case 24 is inserted into a main passage 25 forpassing fluid through. In FIG. 3, air flowing from the air cleaner to anengine (forward flow 40) is directed from an inlet 27 to an outlet 28 ofa secondary passage 28. The secondary passage 28 in a scroll shape isformed by the housing case 24 and the basis 20 as shown in FIG. 4.

The operation principle of the fluid flow sensor 100 is to be describedwith reference to FIG. 1 and FIG. 5. The heating resistor 4 constitutesthe bridge circuit 30 together with the intake air-temperaturecompensating resistor 5, a resistance 10 and a resistance 11. Theresistor 10 and the resistor 11 with respective fixed resistance areformed in the circuit 21 on the basis 20, or on the flow sensing element1. The bridge circuit 30 is put to feed back control so that atemperature of the heating resistor 4 is higher by a predeterminedtemperature than that of the intake air-temperature compensatingresistor 5. Since the heating resistor 4 generates heat depend on theheat insulative property of the thin film 16, the temperature is highestnear the center of the thin film 16 and a temperature distribution of anelliptically radial profile as shown in FIG. 6 is formed. Four thermalsensitive resistors comprising upstream side thermal sensitive resistors6, 7 and downstream side thermal sensitive resistors 8, 9 constitute thebridge circuit 31. A voltage Vref is applied to the bridge circuit 31.The upstream side thermal sensitive resistors 6, 7 and the downstreamside thermal sensitive resistors 8, 9 are heated to a predeterminedtemperature by the heat conduction and heat transfer from the heatingresistor 4. In a windless state, since the upstream side thermalsensitive resistors 6, 7 and the downstream side thermal sensitiveresistors 8, 9 are theoretically heated identically, a difference oftemperature between them is substantially at zero. Accordingly, centertaps 32 and 33 are substantially at an equal potential. In a case wherethe air flow rate exists, since the upstream side thermal sensitiveresistances 6, 7 are cooled and their average temperature are lowered,namely their resistance value are decreased. The downstream side thermalsensitive resistance 8, 9 is little cooled because heated air flowsthrough the down stream side resistance. Accordingly, the upstream sidethermal sensitive resistances 6, 7 become lower than that of thedownstream side thermal sensitive resistor 8, 9, and a temperaturedifferent is produced in correspondence with the air flow rate. Thereby,a potential difference appears between the center tap 32 and the centertap 33. The potential difference is inputted to the micro-processingunit 36. A voltage relative to the air flow rate is outputted from themicro-processing unit 36.

Then, the resistance value of a resistor is generally represented as:R=ρ×L/Ain which R: resistance, ρ: specific resistivity, L: length, A: crosssectional area.

When distortion occurs to the resistance, the change coefficient ofresistance is represented generally as:dR/R=dL/L−dA/A+dρ/ρIn the resistor, the cross section expands or shrinks due to Poisson'sratio of the material relative to expansion or shrinkage in onedirection. Accordingly, assuming Poisson's ratio as ν, it is defined asdR/R=(1+2ν)×dL/L+dρ/ρIn the right side of the formula described above, the first termrepresents the change coefficient due to geometrical deformation, andthe second term represents the effect due to the change of the physicalproperty, which is known as a piezoresistance effect in semiconductormaterials.

The resistance value and the gauge factor of the polycrystalline siliconas the resistor material used for the present invention are determinedby the impurity concentration. Generally, in the polycrystallinesilicon:

-   (1) the resistivity is lower and the gauge factor is smaller in a    case where the impurity concentration is higher, and-   (2) the resistivity is higher and the gauge factor is larger in a    case where the impurity concentration is lower.

In this case, it is necessary to adopt a high resistance value for theupstream side and downstream side thermal sensitive resistors. Because,if their resistance value are low, current flowing through thoseresistors increase, each self heat generation amount thereof increases.In order to suppress the self heat generation amount of the thermalsensitive resistors to such an extent as can be used sufficiently as thefluid flow sensor, it is desirable to restrict their current values toabout 0.5 mA or less. In this case, when 5 V is applied as Vref, it isnecessary that each of the thermal sensitive resistors is about 5,000 Ωor more. Accordingly, for forming a thermal sensitive resistor on arestricted thin film, it is necessary that the width of the thermalsensitive resistor is about 3 to 10 μm. While the self heat generationamount of the thermal sensitive resistor can be suppressed by loweringVref, the difference voltage appearing between the center tap 32 and thecenter tap 33 is decreased and, accordingly, the sensitivity of thefluid flow sensor is lowered, which is not effective means.

In a case where the impurity concentration is increased in order todecrease the change in resistance due to distortion, the resistivity isalso decreased, so that it is necessary to elongate and narrow theresistor pattern shape to make the resistance value higher. Therefore,the resistance tends to undergo the effect of distortion in view of theshape. On the other hand, in a case of lowering the impurityconcentration in order to increase the resistivity, while the shape ofthe resistor pattern can be made wide and short, since the gauge factoris also increased, it also tends to undergo the effect due todistortion. Accordingly, the upstream side and downstream side thermalsensitive resistors tend to undergo the effect of distortion in any ofthe structure.

In a case where distortion occurs in the thin film 16 as the diaphragm,the resistance value of the polycrystalline silicon changes due to thepiezoresistance effect as described above and, particularly, fourthermal sensitive resistors in a fine straight line pattern undergo amost significant effect.

Since the four thermal sensitive resistor 6, 7, 8, and 9 form the bridgecircuit 31, the voltage difference appearing between the center tap 32and the center tap 33 has substantially the same value as in the casewhere the thin film 16 is not deformed when the amount of the resistancechange of the four thermal sensitive resistors 6, 7, 8, and 9 is quiteidentical with each other, so that change of output scarcely occurs.However, in a case where the amount of resistance change is differentamong the four thermal sensitive resistors 6, 7, 8, and 9, since thepotential difference appearing between the center tap 32 and the centertap 33 takes a value different from the case where the thin film 16 isnot deformed, the output is changed to cause change of the flow ratecharacteristics of the fluid flow sensor 100.

The deformation of the thin film 16 as the diaphragm occurs by thestress on the flow sensing element 1. The flow sensing element 1 isbonded to the basis 20, and the basis 20 is bonded to the housing case24. Since a thermosetting adhesive 22 is used for bonding, stress occursinevitably after thermal curing, thereby its effect gives on the flowsensing element 1. Further, while the flow sensing element 1 is mountedin a recess 29 formed to the basis 20, it may be considered a case inwhich the flow sensing element 1 becomes in contact with the wallsurface of the basis 20 that defines the recess 29. The flow sensingelement 1 undergoes stress also in this case. Further, when the flowsensing element 1 and the circuit connection wires areresin-encapsulated, stress exerts an effect on the flow sensing element.Further, in the present embodiment, since the substrate of the flowsensing element 1 is formed of single crystal silicon, the basis 20 isformed of glass ceramics and the housing case 24 is formed of a plasticmaterial, they have different linear expansion coefficients,respectively. Since the environmental temperature of automobiles changewithin a range of about −30 to 130° C. as described above, the flowsensing element 1, the basis 20, and the housing case 24 are deformed byexpansion and shrinkage in accordance with the environmentaltemperature. Accordingly, the thin film is changed, for example, byrelaxation of the stress effectuated initially on the flow sensingelement 1. Such stress relaxation varies depending on the amount of theadhesives, the position and the state of adhesion for each of parts,etc. Thereby, the varieties of deformation and the amount of deformationof the thin film 16 also vary, for example, in the longitudinal,lateral, or twisting direction. Accordingly, the amount of theresistance change tends to take different values in the four thermalsensitive resistors 6, 7, 8, and 9, respectively.

Now, the subject in the prior art is to be described. FIG. 7 shows aschematic view of a thin film pattern in the flow sensing elementdescribed in the Patent Document 1, and FIG. 8 shows a schematic viewfor a driving circuit in the Patent Document 1. In the conventional art,each of the upstream side thermal sensitive resistors 6, 7 anddownstream side thermal sensitive resistors 8, 9 has a curved shapewhich includes more components vertical to the forward flow 40 than thecomponents parallel with the forward flow 40 of airflow. The gaugefactor is different between the longitudinal direction and the lateraldirection for the piezoresistance effect, and the direction (sign) ofchange of resistance is also different in polycrystalline silicon.Accordingly, since the flow sensing element of the prior art includesmore component in one direction (direction perpendicular to the forwardflow 40) of the resistance, the absolute value for the amount of changeincreases. In this case, voltage change tends to occur due to the changeof the resistance of the thermal sensitive resistor at the center tap 32and the center tap 33 and, accordingly, flow rate error occurs due tothe deformation of the thin film. Further, when complicate deformationsuch as twisting occurs in the thin film, and a specified resistance isdeformed in the four thermal sensitive resistors, an extremely largeflow rate error occurs.

On the contrary, in the present invention, the effect due to thedeformation of the thin film described above can be decreased greatly.The structure for decreasing the effect of the deformation is to beshown below.

In the first embodiment of the present invention, each resistor trace asa pattern element of the upstream side thermal sensitive resistors 6, 7,and the downstream side thermal sensitive resistors 8, 9 is configuredso that: (1) the sum for the pattern length in the flowing direction(direction X) and (2) the sum for the pattern length in the directionperpendicular to the flow (direction Y) are substantially equal witheach other. In the polycrystalline silicon, since the sign for the gaugefactor is opposite between the longitudinal direction and the lateraldirection as described above, such structure can easily mutually cancelchanges of resistances in X, y directions due to deformation of thethermal sensitive resistor. Since it cannot be estimated how the thinfilm 16 deforms in the range of variation as described above, theconfiguration of making the length equal between the longitudinaldirection and the lateral direction can most effectively decrease theabsolute value for the amount of changes of the resistances.

FIGS. 9( a) to (c) show other embodiment where a trace as a patternelement of a thermal sensitive resistor is different from that in FIG.1, but the sum for the length of the resistor trace portion as in thedirection X is substantially equal with the sum for that in thedirection Y, as is the case with FIG. 1. Generally, in the semiconductorproduction process, the accuracy of a photomask for conducting thepatterning, and the fabrication accuracy for the resistance is worsenedmore in the curved portion rather than in the straight portion.Accordingly, in a case of including many curved portions as shown inFIG. 9( a), since resistance values of the four thermal sensitiveresistors 6, 7, 8, and 9 vary, one to each other, due to unevenness ofquality in the manufacture, the variations of the characteristics of thefluid flow sensor due to the manufacture may possibly vary greatly.Accordingly, in a case of forming the pattern shown in FIG. 9( a), it isnecessary to sufficiently ensure the photomask accuracy and thefabrication accuracy, which increases the production cost.

Further in resistor trace as a pattern element shown in FIG. 9( b),resistor trace portions elongating in the direction X (trace potionunits in the direction X) and resistor trace portions elongating in thedirection Y (units in the direction Y) of each thermal sensitiveresistor are paired to constitute a thermal sensitive resistor. With thepattern of FIG. 9( b), the number of curved portions can be decreased toless than that in FIG. 9( a). However, there is a limit a size of thethin film, if thin film 16 is extended under such a limit, it ispossible to occur a difference of the deformation between the tracepotion units in the direction X and the trace potion units in thedirection Y, and a flow rate error occurs in this case.

From the foregoings, in a case of making the length of the trace potionsin the direction X and the length of trace potions in the direction Ysubstantially equal with each other, adopting the square-scroll traceshown in FIG. 9( a) and FIG. 1 is effective at suppressing theunevenness in quality of the manufacture. That is, in FIG. 9( a) andFIG. 1, the square-scroll trace is configured by integrating an inwardsquare-scroll portion and an outward square-scroll portion which areconnected with at the center. Further according to such a configuration,since the area of each thermal sensitive resistor can be decreased, theflow rate error due to the deformation in the thin film can be ever-moredecreased.

FIG. 10 shows a pattern for the thin film of a flow sensing elementshowing a second embodiment of the present invention. The thin film 16has a thickness of about 2 to 3 μm and, as the distance is smaller inthe shorter direction of the thin film 16, the strength is higher andthe amount of deformation can be ever-more decreased. As describedabove, in a case where the distance in the shorter direction is small,it is difficult to constitute the square-scroll shape shown in FIG. 1 inview of the area for the thin film. In such a case, it is effective toadopt a configuration as shown in FIG. 9. That is, the trace widths ofthe thermal sensitive resistor potions in the shorter direction and thelongitudinal direction are different from each other, but the ratio ofeach trace width and trace length is equal between the shorter directionand the longitudinal direction. Assuming the trace width in thelongitudinal direction as Wa, the trace width in the shorter directionas Wb, the sum for the trace length in the longitudinal direction as La,and the sum for the trace length in the shorter direction as Lb, sincethe resistance component in the longitudinal direction and theresistance component in the lateral direction can be made equal witheach other by constituting such that La/Wa and Lb/Wb are equal with eachother, the same effect as that in the first example can be obtained.

Incidentally, the prior art also has a problem that the flow ratecharacteristics of the sensor change when the environmental temperaturechanges. This is to be described below. The thin film 16 of the flowsensing element 1 is formed with no distortion at an initial state afterproduction the fluid flow sensor. However, when the fluid flow sensor100 is driven, and the heating resistor 4 generates heat at 100° C. orhigher, a temperature distribution as shown in FIG. 6 is formed to thethin film 16 during a windless state. The thin film 16 causes expansivedeformation in accordance with the temperature distribution. In a case,where the heating resistor 4 is in a shape symmetrical with respect tothe center of the thin film 16 as shown in the conventional example inFIG. 7, the deformation of the thin film 16 due to the heat generationof the heating resistor 4 is substantially symmetrical with respect tothe center of the thin film 16. Accordingly, the amount of deformationof the four thermal sensitive resistors, which are disposed on bothsides as adjacent upstream and adjacent downstream of the heatingresistor 4, are substantially equal with each other to decrease theresultant error of flow rate. However, as described in Japanese PatentLaid-Open No. 2003-83788 and in the present embodiment, in a case wherethe heating resistor 4 is in a U-shaped configuration, deformation ofthe thin film 16 upon heat generation of the heating resistor 4 isasymmetrical with respect to the center of the thin film 16. Thereby, adifference tends to occur for the amount of deformation of the fourthermal sensitive resistors 6, 7, 8, and 9 disposed on both sides of theheating resistor 4. Accordingly, flow rate error tends to occur. A partof heat from heating resistor 4 is released via heat conduction in thewire lead region of the heating resistor 4. Therefore, the amount ofheat received from the heating resistor 4 is different between theregion where the upstream side thermal sensitive resistor 6 and thedownstream side thermal sensitive resistor 8 are disposed and a regionwhere the upstream side thermal sensitive resistor 7 and the downstreamside thermal sensitive resistor 9 are disposed, so that the amount ofdeformation in the thin film is different depend on respective regionsof the thermal sensitive resistors. Further, in a case where the flowrate exists, since temperature difference occurs between the upstreamside thermal sensitive resistors 6, 7 and the downstream side thermalsensitive resistors 8, 9, the amount of deformation of the thin film isdifferent. As described above, in a state where the fluid flow sensor100 is operated, the amount of deformation of the four thermal sensitiveresistors 6, 7, 8, and 9 are different respectively and, accordingly,the amount of resistance change is also different. In an automobile,since the environmental temperature in an engine room changes within arange from −30 to 130° C., the temperature distribution on the thin film16 also changes in accordance therewith. Accordingly, since the amountof deformation of the thin film 16 changes depending on theenvironmental temperature, the characteristics of flow rate change inaccordance with the environmental temperature.

Also for the subject described above, the amount of change ofcharacteristics can be decreased according to the first or secondembodiment of the present invention. Further, with the configurationshown in FIG. 11, change of characteristics due to the change of theenvironmental temperature can be decreased more than that in the firstand the second embodiment of the invention. Configuration of FIG. 11 isto be described. In FIG. 11, each of the thermal sensitive resistorshave three square-scroll trace potions 19 as pattern elements, in whichthe square-scroll trace potions 19 for the upstream side thermalsensitive resistor 6 and the upstream side thermal sensitive resistor 7are arranged alternately. The square-scroll trace potions for thedownstream side thermal sensitive resistor 8 and the downstream sidethermal sensitive resistor 9 are also arranged alternately. Theconfiguration has an effect of making the average temperature for theupstream side thermal sensitive resistor 6 equal with the averagetemperature of the upstream side thermal sensitive resistor 7, and aneffect of making the average temperature for the downstream side thermalsensitive resistor 8 equal with the average temperature of thedownstream side thermal sensitive resistor 9. Accordingly the potentialdifference between the center tap 32 and the center tap 33 less changesand the change of the characteristics of flow rate can be decreased.Further, this is effective also for the deformation of the thin film dueto the stress relaxation described previously. The difference for theamount of resistance change generated in each of the thermal sensitiveresistors 6, 7, 8, and 9 is decreased to less than that in the first andthe second examples of the invention. In this invention, while each ofthe thermal sensitive resistors has three square-scroll trace potions19, they may be disposed by plural numbers other than three depending onthe shape of the thin film 16.

FIG. 13 and FIG. 14 show a fourth embodiment of the present invention.In the fourth embodiment, thermal sensitive resistors 6, 7, 8, and 9 arein a two-layered structure interposing insulating layer betweenresistors 6, 8 and resistors 7, 9 as shown in the cross sectional viewof FIG. 14. In the embodiment, an upstream side thermal sensitiveresistor 7 and a downstream side thermal sensitive resistor 9 are formedin a lower layer, while an upstream side thermal sensitive resistor 6and a downstream side thermal sensitive resistor 8 are formed in anupper layer. These thermal sensitive resistors have also eachsquare-scroll trace potion for a resistor pattern. Further, it isconfigured so that each of the thermal sensitive resistors 6, 8 in theupper layer and each of the thermal sensitive resistors 7, 9 in thelower layer substantially overlap with each other as viewed from theupper surface. With such a configuration, in a case where the thin film16 is deformed, since the amount of deformation is quite identicalbetween the upstream side thermal sensitive resistor 6 and the upstreamside thermal sensitive resistor 7, the amount of change in resistance isalso quite identical with each other. Further, this is applicable alsoto the downstream side thermal sensitive resistor 8 and the downstreamside thermal sensitive resistor 9. Accordingly, since the amount ofchange is quite identical for the thermal sensitive resistors 6, 7, 8,and 9, this provides a configuration with least flow rate error due tothe deformation of the thin film, compared with the first to thirdembodiments. Further, since the area of the thermal sensitive resistors6, 7, 8, and 9 on the thin film 16 can be decreased, the area for thethin film 16 can be decreased and it is possible to improve thestrength, decrease the deformation amount of the thin film, and decreasethe size of the flow sensing element 1. Further, since each of thethermal sensitive resistors 6, 7, 8, and 9 comprises an electricconductor and, accordingly, they have a structure of interposing theinsulating layer between conductors to form a capacitor and improve theresistance to electromagnetic waves.

FIG. 12 shows a system diagram of an internal combustion engine such asa gasoline engine. Air to be taken into the engine flows through apassage comprised of an air cleaner 102, a main passage 25, a throttleangle sensor 103, an idle speed control valve 104, a throttle body 105,and an intake manifold 106. In this air flow course, air flow rate andthe flow direction are sensed by the fluid flow sensor 100 applied withthe present invention. Sensed signals are taken in a vehicle controlunit 107 as voltages or frequencies.

The flow rate signal is used for controlling a combustion sectioncomprising an injector 108, a rotational speed meter 109, an enginecylinder 110, an exhaust manifold 111, and an oxygen densitometer 112and a sub-system.

Although not illustrated, the basic configuration thereof in a dieselsystem is substantially identical with that of the gasoline system, andthe fluid flow sensor of the present invention is applicable thereto.

Further, this technique is applicable also to a fluid flow sensor formeasuring a fluid such as air or hydrogen in a system using a fuel cell.

According to those embodiments, the thermal sensitive resistors formedin the thin film have the following pattern. In a case where the traceas a pattern element of each thermal sensitive resistor has straightline potions with an identical width and with turns in two or moredirections, the sum for the trace lengths of straight line potions inone direction is substantially equal to that in another direction.

On the other hand, in a case where the trace as a pattern element ofeach thermal sensitive resistor has straight line potions with turns intwo or more directions and with different widths according to therespective direction, the following specifications are set. Assuming thewidth of each straight line in each of the directions as W and the sumfor the trace lengths of straight line potions in each direction as L,L/W is substantially equal between the respective directions. With sucha configuration, since the amount of change in resistance value for eachof the thermal sensitive resistors is equal in a case where the thinfilm is deformed. It is capable of suppressing variance of the output ofthe fluid flow sensor. Since fluid measurement with high speed responseand as far as low flow rate region is possible by the fluid flow sensoraccording to the present invention, control at high accuracy is possibleby applying this to an internal combustion engine also under variousrunning conditions. Further, by arranging the thermal sensitiveresistors so that they less suffer from the effect of the temperaturedistribution of the heating resistor, it can provide an effect that theflow rate error less occurs even when the environmental temperaturechanges.

1. A thermal type fluid flow sensor comprising: a heating resistorformed on a thin film of a substrate, and plural thermal sensitiveresistors configuring a bridge circuit and being disposed on the thinfilm of the substrate so as to be located on an adjacent upstream sideand an adjacent downstream side of the heating resistor in a streamdirection of fluid to be measured, wherein a resistor trace for each ofthe thermal sensitive resistors is formed from a combination of twodifferent direction-trace portions capable of substantially cancelingout changes of their resistances to distortion caused in the thin filmto each other.
 2. The fluid flow sensor according to claim 1, wherein aresistor trace for each of the thermal sensitive resistors is formed sothat a change in resistance in one direction and a change in resistancein another direction substantially perpendicular to said one direction,resulting from distortion caused in the thin film, become substantiallyequal with each other.
 3. The fluid flow sensor according to claim 2,wherein the thin film of a substantially rectangular shape is formed onthe substrate, the heating resistor is formed on the thin film byinterposing an insulating layer between the heating resistor and thethin film, the one direction is a longitudinal direction of the thinfilm, and the perpendicular direction is a shorter direction of the thinfilm.
 4. The fluid flow sensor according to claim 1, wherein a resistortrace for each of the thermal sensitive resistors is comprised of pluralsquare-scroll trace portions being connected in series.
 5. The fluidflow sensor according to claim 1, wherein the plural thermal sensitiveresistors are disposed by two on the adjacent upstream side of theheating resistor and by two on the adjacent downstream thereof, and thethermal sensitive resistors are formed of polycrystalline silicon dopedwith impurities.
 6. The fluid flow sensor according to claim 1, whereinthe heating resistor is included to a second bridge circuit differentfrom aforementioned bridge circuit of the thermal sensitive resistors.7. A fluid flow measurement device having a main fluid passage, and asecondary passage for measuring fluid flow rate, comprising: thesecondary passage having a curve; a fluid flow sensor according to claim1 disposed in the secondary passage; and a processing circuit forprocessing signals from the fluid flow sensor element.
 8. A thermal typefluid flow sensor comprising: a heating resistor formed on a thin filmof a substrate, and plural thermal sensitive resistors configuring abridge circuit and being disposed on the thin film of the substrate soas to be located on an adjacent upstream side and an adjacent downstreamside of the heating resistor in a stream direction of fluid to bemeasured, wherein the thermal sensitive resistors are configured by saidthermal sensitive resistors according to claim 1, and are disposedsymmetrically relative to the heating resistor and connectedelectrically to each other to be formed as the bridge circuit.
 9. Thefluid flow sensor according to claim 8, wherein the thermal sensitiveresistors disposed symmetrically are formed as a square-scroll shaperespectively.
 10. The fluid flow sensor according to claim 8, whereinthe plural thermal sensitive resistors are disposed by two on theadjacent upstream side of the heating resistor and by two on theadjacent downstream thereof, and the thermal sensitive resistors areformed of polycrystalline silicon doped with impurities.
 11. The fluidflow sensor according to claim 10, wherein the thin film of asubstantially rectangular shape is formed on the substrate, the heatingresistor is formed on the thin film interposing an insulating layerbetween the heating resistor and the thin film, the one direction is alongitudinal direction of the thin film, and the perpendicular directionis a shorter direction of the thin film.
 12. The fluid flow sensoraccording to claim 8, wherein the heating resistor is included to asecond bridge circuit different from aforementioned bridge circuit ofthe thermal sensitive resistors.
 13. An airflow measurement device foran internal combustion engine having a main airflow passage, and asecondary passage for measuring airflow rate, comprising: the secondarypassage having a curve; a fluid flow sensor according to claim 8disposed in the secondary passage; and a processing circuit forprocessing signals from the fluid flow sensor element, wherein the inletand the outlet of the secondary passage are disposed being inserted inthe main air flow passage as an intake pipe for the internal combustionengine.
 14. A control device for an internal combustion engine forcontrolling the amount of fuel fed to the internal combustion enginebased on signals from the airflow measurement device according to claim13.
 15. A thermal type fluid flow sensor comprising: a heating resistorformed on a thin film of a substrate, and plural thermal sensitiveresistors configuring a bridge circuit and being disposed on the thinfilm of the substrate so as to be located on an adjacent upstream sideand an adjacent downstream side of the heating resistor in a streamdirection of fluid to be measured, wherein a resistor trace for each ofthe thermal sensitive resistors is formed so that the sum for thelengths of trace-portions in one direction is substantially equal withthe sum for the length of trace-portions in another directionperpendicular to said one direction, and these resistor trace portionshave substantially identical widths.
 16. The fluid flow sensor accordingto claim 15, wherein a resistor trace for each of the thermal sensitiveresistors is comprised of plural square-scroll trace portions beingconnected in series.
 17. The fluid flow sensor according to claim 15,wherein the plural thermal sensitive resistors are disposed by two onthe adjacent upstream side of the heating resistor and by two on theadjacent downstream thereof, and the thermal sensitive resistors areformed of polycrystalline silicon doped with impurities.
 18. The fluidflow sensor according to claim 15, wherein the heating resistor isincluded to a second bridge circuit different from aforementioned bridgecircuit of the thermal sensitive resistors.
 19. The fluid flow sensoraccording to claim 15, wherein the thin film of a substantiallyrectangular shape is formed on the substrate, the heating resistor isformed on the thin film interposing an insulating layer between theheating resistor and the thin film, the one direction is a longitudinaldirection of the thin film, and the perpendicular direction is a shorterdirection of the thin film.
 20. A fluid flow measurement device having amain fluid passage, and a secondary passage for measuring fluid flowrate, comprising: the secondary passage having a curve; a fluid flowsensor according to claim 15 disposed in the secondary passage; and aprocessing circuit for processing signals from the fluid flow sensorelement.